1. Field of the Invention
The present invention pertains to accelerometers and, more particularly, to electrodynamic accelerometers including electric field and electromagnetic devices.
2. Description of the Related Art
To date, in most applications, tilt or inclination is usually measured using one of two primary types of sensors. The first type involves the use of bubble type tilt sensors in which a lighter specific gravity fluid, sometimes air, is floated upon a heavier specific gravity fluid. If these two fluids exhibit different electrical parameters, the location of the fluid interface relative to a fixed point on the sensor can be measured electrically and the resultant electrical output can be indicative of the tilt of the sensor. The other primary type of sensor used to measure inclination is an “accelerometer.” Most accelerometers use a proof mass to measure the force required to keep the mass in a fixed or nearly fixed position. These accelerometers are generally only sensitive to acceleration in one axis.
Thus, accelerometers are often used for the measurement of acceleration and deceleration in a variety of applications. Some of the most notable are automotive applications where acceleration measurements are used to initialize deployment of an air bag in the event of sudden deceleration. In these applications, the acceleration range can be on the order of ±50 Gs peak. However, some applications call for measurements on a much smaller scale, which are difficult to make accurately with these types of accelerometers. Consider, for instance, applications where the acceleration of gravity is the measured parameter and the desired result is the determination of tilt or inclination of a measurement platform relative to vertical. In these applications, the nominal acceleration range is on the order of ±1 G and the required resolution of the sensor can be on the order of a few milli-Gs.
Accelerometers can be designed as either open-loop or closed-loop. In an open-loop accelerometer, the proof mass is suspended from a reference point generally using some type of spring. Either the deflection of the proof mass relative to the reference point or the spring stress is measured and indicative of the acceleration. Closed loop accelerometers are similar to open-loop designs in that they use a suspended proof mass and they have a means to measure the deflection of the proof mass when acceleration is applied. Closed loop accelerometers differ from open-loop designs in that they have a means by which a force can be applied to the proof mass to oppose the acceleration forces and maintain the proof mass in a nearly fixed position. The force required to maintain the proof mass in the nearly fixed position is indicative of the acceleration.
Consider the test fixture 100 in FIG. 1. If a solid dielectric plate 103 is placed between two electrode plates 106, and a voltage is applied to the electrode plates 106, a force, Fc, will be exerted on the dielectric plate 103. The force Fc will tend to center the dielectric plate 103 between the electrode plates 106 as shown in ghosted lines 109. This centering force, Fc can be determined. In this example, the dielectric plate 103 is assumed to have a relative dielectric of K; and, the relative dielectric of the void space surrounding the plate is assumed to be that of free space e0, or 1. A voltage, VS, is applied to the plates. The lengths, L, of the electrode plates 106 and dielectric plate 103 are equal. The electrode plates 106 have a width of b (dimension not shown). The electrode plates 106 are separated by a distance, d. The thickness of the dielectric plate 103 is slightly less than the plate separation distance, d. The centering force, FC, is defined as:
                              F          C                =                                                            V                s                2                            ⁢                              e                0                            ⁢              b                                      2              ⁢              d                                ⁢                      (                          K              -              1                        )                                              (        1        )            where                VS=voltage applied to the plates;        e0=permittivity of free space;        b=width of the plates;        d=spacing between the plates; and        K=relative dielectric constant of the dielectric plate.As an example, the voltage applied to the apparatus and the relative dielectric of the dielectric plate 103 are assumed to be as follows:        VS=30 V (or J/coulomb);        e0=8.85E-12 coulomb2/N-m2;        b=0.1 inch (or 0.00254 m);        d=0.01 inch (or 0.000254 m); and        K=500.The centering force Fc is then calculated to be:FC=1.99E-05 Newton  (2)The capacitance measured between the plates is defined as:        
                    C        =                                                            e                0                            ⁢              bL                        D                    ⁢          K                                    (        3        )            
Assume now that K1 is defined as the initial relative dielectric constant of the region between the electrode plates 106. In the previous example, K1 was defined to be the relative dielectric of free space e0, or 1. C1 is defined as the initial capacitance measured between the electrode plates 106. K2 is defined as the relative dielectric of the dielectric plate 103. C2 is defined as the new capacitance measured when the dielectric plate 103 is fully centered. Eq. (1) can now be rewritten to a form which describes the centering force as a function of the capacitance change as follows:
                              F          C                =                                            V              s              2                                      2              ⁢              L                                ⁢                      (                                          C                2                            -                              C                1                                      )                                              (        4        )            
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.